1. Field of the Invention
This invention relates in general to power line carrier communication systems and, more specifically, to signal couplers for use in such communication systems.
2. Description of the Prior Art
Power line carrier communications have been carried over high voltage transmission lines between the power generating site and the power transmission switching site or distribution substation site for many years. The recent increase in the desirability of performing certain distribution functions, such as automatic reading of utility meters, selective load control, and the like, has made it desirable to use the distribution power line conductors for the transmission of communication signals. However, the use of the distribution network conductors for the transmission of carrier communication signals presents many problems not encountered in high voltage transmission line communication systems. Some of these problems include the poor high frequency impedance characteristics and the high level of electrical noise present on the distribution network conductors which, along with the plurality of distribution transformers and power factor correction capacitors attached to the distribution network, rapidly attenuate the communication signals.
In general, a distribution network communication system includes a central communication terminal which transmits a frequency modulated interrogation signal to a remote communication terminal, usually located at a customer premise. The remote communication terminal, on receiving the interrogation signal, performs the desired automation function and sends a response signal containing the automation function data back to the central communication terminal.
Many different distribution network communication systems have been developed to transmit communication signals between the central communication terminal and the remote communication terminal. U.S. Pat. No. 3,702,460, issued to J. Blose, and patent application Ser. No. 649,774, filed Jan. 16, 1976, in the name of L. Vercellotti, now U.S. Pat. No. 4,016,429, and assigned to the assignee of the present invention, disclose a distribution network communication system which utilizes the grounded neutral conductor of the distribution network for signal communication. The central communication terminal and the remote communiation terminals are disposed in signal communication with the grounded neutral conductor of the distribution network by signal couplers comprised of a hollow magnetic core surrounding the neutral conductor to form a single turn primary winding configuration. Furthermore, capacitive elements placed in parallel relationship with the ferritic core form a parallel resonant circuit at the frequency of the communication signals. Signal repeaters are coupled to the neutral conductor by similar means at spaced intervals along the distribution network in order to amplify and improve the signal to noise ratio of the communication signal.
Another type of distribution network communication system is disclosed in U.S. Pat. No. 3,911,415, issued to I. Whyte, and assigned to the assignee of the present invention, wherein the primary and secondary conductors of the distribution network are utilized as a signal transmission path. In this type of communication system, signal repeaters, in addition to amplifying the communication signal, are used at each distribution transformer location to transfer the communication signal between the primary and secondary conductors and thereby bypass the transformer which is the primary source of signal attenuation in the distribution network. Since the signal repeaters in this type of communication system are coupled to the high voltage primary conductors, a high voltage signal coupler is required. The high voltage signal couplers utilized in prior art communication systems of this type, are typically comprised of a high voltage coupling capacitor and a drain coil or matching transformer; wherein the coupling capacitor connects the signal repeater to the high voltage primary conductor of the distribution network and the drain coil or matching transformer provides impedance matching for the receiver and transmitter sections of the signal repeater. The most common type of coupling capacitor used in prior art communication systems includes two bushings which electrically isolate the high and low voltage terminals of the capacitor from the capacitor housing. One of the bushings connects the high voltage capacitor terminal to the primary conductor, while the other bushing connects the low voltage terminal to the drain coil or matching transformer. The drain coil or matching transformer is in turn connected to ground via a ground conductor which is mounted on each utility pole or vertical support and connects the neutral conductor of the distribution network to real or earth ground.
While this type of signal coupler provides satisfactory signal coupling, various portions of the circuit can rise to the voltage of the primary conductor if the ground connection is broken. For example, the normally low voltage terminal of the capacitor can rise to the potential of the primary conductor if the drain coil or matching transformer becomes disconnected from ground or if its winding becomes open-circuited. Furthermore, the normally grounded terminal of the drain coil or matching transformer can rise to the voltage of the primary conductor if the terminal is removed or disconnected from real ground. Also, the use of twin bushing capacitors is also quite expensive and in distribution systems above 10 kV become almost impossible to obtain. Less expensive single bushing capacitors, in which the low voltage terminal of the capacitor is connected to the capacitor housing have also been used in the past. However, single bushing capacitors cannot be used in some distribution networks in which the utility company requires that the ground conductor must be able to be seen by the linemen to be continuous and unbroken. Since all capacitor cases are considered to be at real ground potential, the single bushing capacitor cannot be utilized in prior art signal couplers which connect the capacitor to the drain coil instead of to ground. Thus, it becomes desirable to provide a signal coupler for use in a communication system utilizing the high voltage primary conductors of a distribution network for the transmission of carrier communication signals in which the less expensive single bushing coupling capacitor can be utilized, the signal coupler can be easily installed in existing distribution networks and the ground connection can easily be observed to be continuous.